Camera eyepiece light compensation arrangement

ABSTRACT

A camera having circuitry capable of compensating for light entering through an eyepiece of the camera, so that the exposures will not be rendered inaccurate due to light entering through the eyepiece. The circuitry is capable of measuring the light entering through the objective of the camera as well as through the eyepiece of the camera, and the total of the light entering through the objective and eyepiece is converted into an electrical quantity in the form of a collector current of a transistor. Also, the circuitry measures only the light entering through the eyepiece and converts this light into an electrical quantity in the form of the collector current of a second transistor. The electrical circuitry has conductors which direct these collector currents to a common junction to which there is connected an output conductor for conducting an electrical quantity in the form of an output current equal to the difference between the collector current corresponding to the total light entering through the objective and eyepiece and the collector current corresponding to the light entering only through the eyepiece, so that in this way the output current corresponds only to the light entering through the objective and can be used in the determination of the extent to which film in the camera is exposed.

United States Patent [1 1 Mori [451 Sept. 17, 1974 CAMERA EYEPIECE LIGHTCOMPENSATION ARRANGEMENT [75] Inventor: Chiharu Mori, Tokyo, Japan [73]Assignee: Asahi Kgalru KogyoKabushiki Kaisha, Tokyo, Japan [22] Filed:May 5, 1971 [21] Appl. No.: 140,512

[30] Foreign Application Priority Data Primary ExaminerJoseph F. Peters,Jr. Attorney, Agent, or FirmSteinberg & Blake [5 7] ABSTRACT A camerahaving circuitry capable of compensating for light entering through aneyepiece of the camera, so that the exposures will not be renderedinaccurate due to light entering through the eyepiece. The circuitry iscapable of measuring the light entering through the objective of thecamera as well as through the eyepiece of the camera, and the total ofthe light entering through the objective and eyepiece is converted intoan electrical quantity in the form of a collector current of atransistor. Also, the circuitry measures only the light entering throughthe eyepiece and converts this light into an electrical quantity in theform of the collector current of a second transistor. The electricalcircuitry has conductors which direct these collector currents to acommon junction to which there is connected an output conductor forconducting an electrical quantity in the form of an output current equalto the difference between the collector current corresponding to thetotal light entering through the objective and eyepiece and thecollector current corresponding to the light entering only through theeyepiece, so that in this way the output current corresponds only to thelight entering through the objective and can be used in thedetermination of the extent to which film in the camera is exposed.

16 Claims, 7 Drawing Figures P813111. measu MH SeeEion l* Opemi:ion-imms section CAMERA EYEPIECE LIGHT COMPENSATION ARRANGEMENT BACKGROUND OFTHE INVENTION The present inventionrelates to cameras.

In particular, the present invention relates to singlelens reflexcameras where exposures are automatically determined utilizing lightwhich has passed through the objective. Thus, the present invention isparticularly designed for through-the-lens (TTL) single lens reflexcameras.

As is well known, with cameras of this type before an exposure isactually made a tiltable mirror serves to reflect the light enteringthrough the objective up to the viewfinder where the object to bephotographed is visible through an eyepiece of the viewfinder. As iswell known, immediately before exposure this mirror is tilted upwardlybeyond the optical axis so that the film can be exposed with lightentering through the objective.

A particular problem encountered with cameras of this type is that lightcannot be prevented from entering into the camera through the eyepieceinasmuch as the light-receiving element is positioned in the light pathof the range finder. Various attempts have been made to eliminate theinfluence of light entering through the eyepiece, such as byutilizingdifferent types of constructions with respect to the positionof the lightreceiving element, the arrangement of the optical system forlight measurement, etc. However, none of the known constructions arecapable of completely eliminating the influence of light which entersthrough the eyepiece. Cameras of the above type which use a stopdowndiaphragm light measurement system are particularly liable toundesirable influence by light which enters through the eyepiece.

SUMMARY OF THE INVENTION It is accordingly a primary object of thepresent invention to provide a camera with a structure which is capableof eliminating the above drawbacks by achieving a complete compensationfor the light which enters through the eyepiece.

In particular, it is an object of the present invention to provide acamera with a structure which does not rely upon mechanical contrivancesto eliminate the influence of eyepiece entering light and instead reliesonly upon translstorized electrical circuitry for this purpose.

Thus, it is an object of the present invention to provide a camera whichis capable of compensating perfectly for eyepiece entering lightirrespective of the conventional mechanical arrangement of the cameracomponents.

It is also an object of the present invention to provide for a cameracircuitry capable of achieving the above objects while at the same timebeing characterized by an extremely small temperature dependency as wellas an extremely small source voltage dependency.

Also, it is an object of the present invention to pro vide aconstruction of this type which is exceedingly simple so that it lendsitself to inexpensive massproduction manufacturing techniques without inany way detracting from the accuracy of the operation of the parts.

The camera has an objective through which light enters to make theexposure as well as an eyepiece through which light also enters into theinterior of the camera. According to the invention a photoresponsivemeans is provided for receiving the light entering through the objectiveand through the eyepiece and for converting this received light into acorresponding electrical quantity. An eyepiece light measuring means isalso provided for measuring only the light entering through the eyepieceand converting this latter light into a corresponding electricalquantity. An electrical circuit means is provided for subtracting thequantity corresponding to eyepiece light from the quantity correspondingto the total light entering through the eyepiece and objective,achieving in this way an output electrical quantity corresponding to thedifference between the eyepiece light and the light which is received bythe photoresponsive means, so that this output quantity can then be usedin the determination of the extent to which film in the camera isexposed.

BRIEF DESCRIPTION OF DRAWINGS The invention is illustrated by way ofexample in the accompanying drawings which form part of this applicationand in which:

FIG. 1 is a schematic diagram of circuitry and components illustratingthe principle of the present invention; and

FIGS. 2-7 respectively illustrate different embodiments of circuitry ofthe present invention particularly suited for single lens reflex camerasof the type which measure internal light for exposure purposes.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring first to FIG. I whichillustrates in particular the principles upon which the presentinvention is based, it will be seen that the illustrated circuitincludes a current source E. The illustrated circuitry also includes aphotosensitive means made up of a pair of light-receiving elements D,andD these elements being photovoltaic elements such as siliconphotocells in the illustrated example. The photovoltaic element D, isarranged to receive light I, entering through the objective indicated bythe schematically represented lens in FIG. 1 as well as light I enteringthrough the eyepiece which is also schematically represented in FIG. 1.On the other hand, the photovoltaic element D receives only light I fromthe eyepiece.

The circuit further includes transistors Q, and Q2 which arerespectively of NPN and PNP types and which respectively have equalelectrical characteristics, with the polarities reversed with respect toeach other, and the transistors are connected in series as shown inFIG. 1. 7

Thus, the photovoltaic element D, receives the light input component Icoming from the object to be photographed through the objective, and thecomponent I entering through the eyepiece. As is well known this lightwill provide through the photovoltaic element D, a voltage V, across theterminals of the light-receiving element D,, and this voltage V, is inproportion to the logarithmic value of the light input intensitycorresponding to the components I and I at the time of the lightmeasurement. The logarithmic compression rate of the photoelectrictransformation characteristic of the light-measuring photovoltaicelement D, is made equal to the logarithmic expansion rate of thebaseemitter voltage vs. the collector current characteristic of thetransistor Q, whose input is the output voltage V,, so that thecollector current i, of the transistor Q, is proportional to the lightinput intensity equal to the total of components I and I at the time oflight measurement. Thus, the transistor Q, can be considered as a typeof constant current source having a constant current characteristic dueto the collector characteristic. Thus it will be seen that thetransistor Q, and the photovoltaic element D, form a photoresponsivemeans ror receiving the light entering through the objective and theeyepiece and for converting this light into an electrical quantity,namely the collector current i,, which corresponds to the receivedlight.

In the same way, the photovoltaic element D forms that part of thephotosensitive means which measures only the light component I and thisphotovoltaic element D which corresponds in all ways to the photovoltaicelement D, is electrically connected in the same way to the transistorO, which is of the same character istics as the transistor Q, so thatthe photovoltaic element D and the transistor 0, form an eyepiece lightmeasuring means for receiving the eyepiece-entering light and convertinginto an electrical quantity in the form of the collector current i whichcorresponds to the eyepiece light. Thus the collector current i, isproportional to the eyepiece-entering light component I and thetransistor may aso be considered as a type of constant current'sourcehaving a constant current characteristic due to the collectorcharacteristic.

As may be seen from FIG. 1, the circuitry includes an electrical circuitmeans having conductors for conducting the collector currents i, and i,to a common junction to which there is also connected a conductor of theelectrical circuit means in which the output current i flows, thisoutput current flowing through the illustrated ammeter A. Of course, thelight component I, will in general be greater than the eyepiece enteringlight component I,,, so that the output current i is proportional onlyto the light input component I,-, corresponding to the light enteringthrough the objective from the object which is to be photographed, andthus the output current i is an electrical quantity corresponding to thedifference between the collector currents i, and i and thuscorresponding to the light component I,-. This output current i which isproportional only to the light input component from the object to bephotographed is a charging current source having a constant currentcharacteristic. This output current i, is utilized as the photoelectrictransformation output of the light input intensity from the object to bephotographed, so that it is possible to utilize this quantity i in anelectric shutter control circuit which determines the extent of exposureof film in the camera. Thus this latter shutter control circuit can beprovided with the electrical quantity i which is completely compensatedfor the eyepiece-entering light influence encountered in TTL single lensreflex cameras.

While in the above example of FIG. 1 the photosensitive means isconstituted by a pair of light-receiving elements D, and D in theembodiments which are respectively illustrated in FIGS. 2-7 there isonly a single light-receiving element which in the embodiments of FIGS.2-6 is a photovoltaic element and which in the embodiment of FIG. 7 is aphotoconductive element. Each of the embodiments of FIGS. 2-7 includes aswitch means which serves to connect the single lightreceiving elementfirst into the circuit which receives the light components enteringthrough the objective and the eyepiece and then into the circuit whichmeasures only the light entering through the eyepiece. In order toretain in the circuit the collector current i, corresponding to thetotal light the totalling means has a memory means for retaining thiscurrent i, while the eyepiece light is measured by the eyepiece lightmeasuring means to provide the collector current i Referring to FIG. 2the light measurement memory section illustrated therein includes thephotosensitive means formed by the single photovoltaic element D. Theillustrated circuit includes the current source E as well as thetransistors Q and G, which are of common characteristics but reversepolarity and which are connected in series with the conductors for thecollector currents i, and i having a common junction to which theconductor for the output current i;, is connected.

The photosensitive means D of FIG. 2 is a P N junction photovoltaicelement across the terminals of which there is the voltage V,, thisphotosensitive means D being a silicon photocell in the illustratedexample. The photosensitive means D is permanently connected between theblades of a double-pole double-throw memory switch SW,, and in the firstposition of this switch means SW, the blades thereof engage a pair ofcontacts a, while in the next position thereof,just prior to swinging upof the mirror of the camera, these blades of the switch means will bedisplaced in a known way into engagement with the contacts b of theswitch means. Thus, when the switch means SW, has the positionillustrated in FIG. 2 for measuring the total of the components I, andI,,, the generated voltage V, is proportional to the logarithmic valueof the intensity of the light input of the components I and I Thecircuit includes the memory capacitor C, which is thus charged by thevoltage V, in the first position of the switch means SW,. Immediatelybefore the mirror is swung up upon operation of the shutter-releaseelement of the camera, the blades of the switch means SW, are displacedaway from the first pair of contacts a and into engagement with thesecond pair of contacts b. The voltage V, acts also as the input voltageto a sourcefollower circuit consisting of the field effect transistor Q,and the resistor R,. so that after the double-pole double-throw switchmeans SW, has its blades displaced into engagement with the second .pairof contacts b. the output voltage V, of the source-follower circuit ismaintained constant. This output voltage V acts as the input to thenext-stage light measurement memory section output transistor 0;, sothat the collector current i, of the transistor 0 has a value determinedby the light input intensity corresponding to the components I and I ofthe photovoltaic element D.

Thus, when the shutter-release button of the camera is actuated and thememory switch means SW is displaced from the first pair of contacts atethe second pair of contacts b immediately before swinging up of themirror, the collector current i, is retained after the mirror has swungup. and the photosensitive means D now receives only theeyepiece-entering light for measuring only the component I Now theoutput voltage V;, of the photosensitive means D acts as the inputacross the base and emitter of the transistor 0,, of the eyepiece lightmeasuring means, so that the collector current i, of the transistor 0;,has a value which correis arranged to coincide with the photoelectricoutput characteristic of the eyepiece-entering light compensationsection consisting of the photovoltaic element D and the transistor 03,So that the output current 1",, of the light measurement memory section,which is equal to the collector current i, of transistor 0,corresponding to the light input intensity corresponding to components Iand I during light measurement minus the collector current i, of thetransistor 0,, determined only by the eyepiece-entering light intensityI,,, has a value corresponding only to the light input intensity Icoming from the object which is to be photographed.

In the operation-timing section illustrated in FIG. 2, the outputcurrent i of the light measurement memory section is logarithmicallycompressed by a logarithmic compression element (1,, so that, as is wellknown, the voltage V, across the terminals of the logarithmiccompression element d, varies linearly with the exponential functionvariation of the light input intensity lp coming from the object to bephotographed. In addition the voltage V, across the terminals of asecond logarithmic compression element d varies linearly with theexponential function variation of the resistance value of a variableresistor R,-. This is a conventional variable resistor which is set to apredetermined resistance corresponding to the combination of theselected aperture of the diaphragm and the film speed.

The voltages V, and V, across the terminals of the logarithmiccompression elements, in the form of suitable diodes, for example. aresupplied to the bases of a pair of computation and logarithmic expansionor extension transistors T, and T, as inputs to the computation andlogarithmic expansion or extension section consisting of the transistorsT, and T, and a resistor r,, so as to carry out the photographiccomputation. The characteristics of the logarithmic compression elementsd, and d,. the computation and logarithmic extension transistors T, andT the resistor r, and the variable resistor for introducing the aperturesize and film speed are so selected that the collector current of thelogarithmic extension transistor T namely the timing charging currenti,, conforms to the Apex index.

In addition, the capacity of the timing capacitor C, and the switch-overlevel of a switching circuit consisting of the transistor T an SCR (1,.resistors r, and r, and electromagnet M are so selected as to conform tothe Apex index. Thus, the exposure time is obtained as the duration fromthe instant when the timing switch SW is displaced from its closed toits open condition. As is well known, this timing switch will operate insynchronism with the leading curtain of the focal plane shutter ofthecamera. When this leading curtain moves across the film gate from theshutter-closing to the shutter-opening position, the switch SW isautomatically opened, so that now the timing capacitor C, becomescharged. In accordance with the switch-over level ofthe switchingcircuit the electromagnet M is actuated, as by being deenergized, forexample, to release the trailing curtain of the camera so that thistrailing curtain is now driven by springs connected thereto in order tofollow the leading curtain and close the shutter, thus terminating theexposure in a well known manner.

In the light measurement memory section illustrated in FIG. 2, thetemperature dependencies of the photovoltaic element D and thetransistor Q, and also the temperature dependencies of the photovoltaicelement D and the transistor 0, cancel each other, so that the lightmeasurement memory circuit is characterized by an extremely smalltemperature dependency and in addition, due to the circuit arrangement,by an extremely small source voltage dependency.

Thus it will be seen that with the embodiment of FIG. 2 thephotosensitive means includes a single photovoltaic element D whichthrough the switch means formed by the double-pole double-throw switchSW, is made common to and forms parts of the photoresponsive means whichreceives light entering through the objective and the eyepiece and theeyepiece light measuring means, this photosensitive means D of FIG. 2serving in the first position of the switch means to provide thecollector current i, and in the second position to provide the collectorcurrent i,, with the circuitry conducting these currents to the commonjunction to which is connected a conductor for the output current i Thememory capacitor C, acts when the blades of the switch means aredisplaced into engagement with the second pair of contacts b to form amemory means to retain the collector current i,.

In the embodiment of the invention which is illustrated in FIG. 3 use ismade of the same operationtiming section the details of which areillustrated in FIG. 2. Therefore, for the sake of simplicity this entiresection is indicated as a single block in the circuit of FIG. 3. Also,in FIG. 3 the photoresponsive means is identical with that of FIG. 2,although a single-pole double-throw type of memory switch means SW, isused in this embodiment. The photosensitive means is also formed by thesingle photovoltaic element D, and the single blade of the switch meansSW, of FIG. 3 is permanently connected to the photosensitive means D ofFIG. 3. Initially during measurement of the light entering through theobjective and the eyepiece the blade of the switch engages the contacta, so that initially the photovoltaic element D is connected across theterminals ofthe capacitor C,, and it will be noted that precisely thesame arrangement is obtained with the embodiment of FIG. 2 when theblades of the switch means thereof are connected to the pair of contactsa in FIG. 2. Thus, the photoresponsive means of FIG. 3 operates in thesame way as that of FIG. 2 for achieving the collector current i, ofthetransistor O, which corresponds to the total of the light enteringthrough the objective and the eyepiece during light measurement. Withthis embodiment when the shutter-release button of the camera isactuated to release the shutter, immediately before the mirror swings upthe single-pole double-throw memory switch means SW, is changed over sothat its blade is displaced from the contact a and instead engages thecontact b. Now the circuit of the photoresponsive means will act toretain the collector current i, in the manner described above. With thephotosensitive means D now connected through the switch means to thecontact b, only the light entering through the eyepiece will be measuredsince the mirror is in its upper position, and thus the eyepiece lightmeasuring means operates at this time to measure only the lightcomponent I The output voltage at this time acts as the input betweenthe base and the emitter of the transistor Q, so that the value of thecollector current i, of

. the transistor 0., corresponds to the eyepiece-entering lightcomponent I,,. This collector current i 2 is logarithmically compressedby the logarithmic compression element D,, and the output voltage isapplied as the input to a transistor 0,, so that the collector currenti, of the transistor Q, has a value corresponding only to and determinedexclusively by the eyepiece-entering light intensity I,,. Thephotoelectric output characteristic of the light measurement memorysection, which consists of the photovoltaic element D, the field effecttype of transistor 0,, the capacitor C,, the resistor R, and thetransistor 0,, is arranged to coincide with the photoelectric outputcharacteristic of the eyepiece-entering light compensation sectionconsisting of the photovoltaic element D, the transistors Q3 and Q4, andthe logarithmic compression element D,, so that the output current i, ofthe light measurement memory section is obtained through the transistorQ, with the current value of the output current 1', corresponding onlyto the light input intensity I coming from the object which is to bephotographed.

In the circuit of FIG. 3 the temperature dependencies of thephotovoltaic element D and the transistor Q and of the photovoltaicelement D and the transistor Q, as well as of the logarithmiccompression element D, and the transistor 0,, all cancel each other.Therefore the entire circuit is characterized by an extremely smalltemperature dependency and also by a very small source voltagedependency as a result of the particular circuit construction. Inaddition, since the switch means SW, is of a single-pole double throwtype, the light measurement memory circuit can be massproduced with verygreat practical advantage.

In the embodiment of FIG. 4, the photoresponsive means and eyepiecelight measuring means both include in this case also a singlephotovoltaic element D which is common to both of these means, and inthe embodiment of FIG. 4 there is a double-pole doublethrow switch meansSW, which initially has the position illustrated in FIG. 4 in order torender the photoresponsive means operative. this photoresponsive meanshaving the memory capacitor C,. Thus during measurement of the intensitymade up ofthe components I, and I the output current, upon shortcircuiting, from the photovoltaic element D, as is well known, will beproportional to the sum of components l,- and I It will be seen that inthe circuit of FIG. 4 the output current i, of the photovoltaic elementD acts as the input to the emitter of the illustrated base-groundedtransistors 0,, which in the illustrated example forms the transistor ofthe eyepiece light measuring means. By connecting the photovoltaicelement D in this way between the base and emitter of the base-groundedtransistor the load ofthe photovoltaic element D is very small andaccordingly the linearity of the output characteristic of thephotovoltaic element D is retained over a wide range, so that a currentwhich is nearly equal to the output current i, is obtained as thecollector current i, of the transistor 0 and it is this collectorcurrent which forms the electrical quantity corresponding to thecomponent I formed by the eyepiece-entering light. It will be seen thatwith the embodiment of FIG. 4 this transistor 0,, also is common to thephotoresponsive means and the eyepiece light measuring means and in theillustrated position of the switch means SW,. the collector current i,is transmitted through the switch SW,, when it has the position shown inFIG. 4, to a logarithmic compression element D, so that in this way thiscollector current i, is logarithmically compressed. The output voltageV, serves to charge the memory capacitor C, across the terminals ofwhich the logarithmic compression element D, is connected when theswitch means SW, has the position shown in FIG. 4. This .output voltagealso acts as an input to the gate of the field effect type transistorQ,. Thus, the collector current i, of the transistor 0, is of a valuecorresponding to the total of the light input components I; and I,, asdetermined by the receiving of both of these components by thephotovoltaic element D in the manner described above.

When the shutter release button of the camera is actuated, the memoryswitch means SW, is changed over from the position illustrated in FIG.4-to the position where the lower blade of FIG. 4 opens the circuit ofthe capacitor C, and the upper blade of the switch means SW, of FIG. 4engages the third contact b. Thus at this time the collector current i,is retained by the memory means formed by part of the circuit of thephotoresponsive means, and with the mirror swung to its upper positionthe photovoltaic element D will receive only the eyepiece-entering lightI As a result the collector current 1', of the transistor Q, has amagnitude which is approximately proportional to the light inputintensity I of the light received at this time by the photosensitivemeans D, and thus the transistor 0,, functions also as the transistor ofthe eyepiece light measuring means.

The photoelectric output characteristic of the eyepiece-entering lightcompensation section formed by the photovoltaic element D and thetransistor Q, is arranged so as to coincide with the photoelectricoutput characteristic of the light measurement memory section formed bythe photovoltaic element D, the transistors Q and 0,, the logarithmiccompression element D,, the capacitor C, and the resistor R,, so thatthe output current i, of the light measurement memory section obtainedby the transistor 0, has a value which is approximately proportional tothe light input component I,- coming only from the object which is to bephotographed. With this particular light measurement memory section ofFIG. 4, the output current of the photo, voltaic element D acts as theinput to the emitter of the base-grounded transistor. so that, ascompared with the embodiments of FIGS. 2 and 3, the connection of thephotovoltaic element D into the electronic circuit is much easier withthe embodiment of FIG. 4. In addition, since the photoelectric outputcurrent is obtained as the collector current of a base-groundedtransistor having a constant current characteristic, it is not necessaryto provide for the circuitry of FIG. 4 an additional transistor foreyepiece-entering light compensation. The transistor Q, acts as thislatter transistor as well as for forming part of the photoresponsivemeans when the switch SW, has the position of FIG. 4 in order totransmit the collector current i, into the circuit of thephotoresponsive means described above.

In addition, the characteristics of the circuitry of FIG. 4 are suchthat the temperature dependencies of the logarithmic compression elementD, and the transistor Q, cancel each other resulting in an extremelysmall temperature dependency of the entire circuit, and the sourcevoltage dependency of this circuit also is extremely small as a resultof the particular circuit construction. In this way there is a greatadvantage in connection with mass-production of a lightmeasurementmemory circuit conforming to the arrangement shown in FIG. 4.

It will be noted that in FIG. 4 the operation-timing section isindicated in the same simplified manner as was the case with FIG. 3, andthe same is true with the illustration in FIG. 5. In the embodiment ofFIG. 5 the single photovoltaic element D also is common to and formspart of the photoresponsive means for receiving the light components Iand I,, and the eyepiece light measuring means for measuring only thelight I,, entering through the eyepiece. However, with the embodiment ofFIG. 5 this photovoltaic element D is utilized as a photodiode. Thisphotovoltaic element D of FIG. 5 is electrically connected with reversebias between the collector and base of a photocurrent amplifyingtransistor 0,. A pair of logarithmic compression elements D, and D, arerespectively connected in series with the emitter and the collector ofthe amplifying transistor 0,. The switch means SW, of this embodiment isin the form of a simple single-pole single-throw switch. When thisswitch means is in the closed position illustrated in FIG. 5 the lightcomponents I F and I,, will be totalled. At this time the emittercurrent of the transistor Q, has a constant current sourcecharacteristic and is ofa value approximately proportional to the lightinput intensity. This emitter current is logarithmically compressed bythe lararithmic compression element D,. The output voltage V, actsthrough the memory switch SW, to charge the memory capacitor C, acrossthe terminals'of which the logarithmic compression element D, isconnected when the switch SW, is in the illustrated closed position.Through a suitable transmission which is actuated by depression of theshutterrelease button of the camera, the mirror is swung up, but justbefore the mirror is swung up the transmission will also serve to-movethe switch means SW, from its closed to its open position. Then afterthe mirror has swung up the collector current i, is retained and thephotosensitive means D receives only the light entering through theeyepiece so as to measure only the component I Therefore at this timethe emitter current of the transistor 0, has a value approximatelyproportional to the light input intensity of component I,,, as in theembodiment of FIG. 4. The emitter current is logarithmically compressedby the logarithmic compression element D The resulting voltage V, actsas the input voltage to the transistor Q of the eyepiece light measuringmeans. Thus, the collector current i, of the transistor Q has a valuedetermined by the eyepiece-entering light component I The logarithmiccompression-expansion characteristic of the light measurement memorysection, which consists of the logarithmic compression element D,, thefield effect type of transistor 0,, the capacitor C,, the resistor R,and the transistor 0 is arranged to coincide with the logarithmiccompression-expansion characteristicof the eyepiece-entering lightcompensation section which consists of the logarithmic compressionelement D and the transistor 0,, so that theoutput current i of thelight measurement memory section obtained by way of the transistor 0,has a value corresponding only to and determined only by the intensityof the light input component I coming from the object which is to bephotographed. With the light measurement memory section of FIG. 5, byconnecting the photovoltaic element D with reverse bias between thecollector and base of the transistor 0,, a high sensitivityphotoelectric transformation output is obtained having a constantcurrent source characteristic of small temperature dependency and alsoof extremely small voltage dependency. Furthermore, the temperaturedependencies of the logarithmic compression element D, and thetransistor 0, as well as of a logarithmic compression element D and thetransistor Q cancel each other. The entire circuit has an extremelysmall source voltage dependency as a result of the constructionillustrated in FIG. 5. In addition, since the switch means SW, is of asingle-pole single-throw type, the light measurement memory circuit canbe mass-produced very readily with great practical advantages.

With the embodiment of FIG. 6, as was the case with the embodiment ofFIG. 5, upon light measurement the transistor 0, causes a collectorcurrent i, to flow, and this collector current corresponds to the totallight input intensity of the components I, and I The switch means SW, ofthis embodiment is a double-pole double-throw type of memory switch asdescribed above and shown in FIG. 4. Thus, just prior to swinging up ofthe mirror so that the circuitry functions only to measure theeyepiece-entering light, the double-pole double-throw switch SW, of FIG.6 has its blades shifted away from the first pair of contacts a so as toopen the circuit of the capacitor C, across whose terminals thelogarithmic compression element D, was initially connected when theswitch is in the position illustrated in FIG. 6. Simultaneously with theopening of the circuit which includes the memory capacitor C, the upperblade of FIG. 6 engages the contact b, so that the collector current i,is retained after the mirror has swung up and now the light-receivingelement receives only the eyepiece-entering light component I,,.so thatthe emitter current i, of the transistor 0, has a value determined onlyby the eyepiece-entering light component I,,. The photoelectric outputcharacteristic determined by the light-receiving element D and thetransistor 0, is arranged to coincide with the photoelectric outputcharacteristic of the light measurement memory section made up of thelight receiving element D, the transistor 0,, the logarithmiccompression element D,, the field effect type of transistor 0,, thecapacitor C, and the transistor 0 so that the output current i, of thelight measurement memory section obtained through the transistor 0, hasa value determined only by and corresponding only to the light input I;coming from the object to be photographed. As compared with theembodiment of FIG. 5, it is possible for the embodiment of FIG. 6 tofunction without any special use of the eyepiece-entering lightcompensation circuit section, so that the circuit of FIG. 6 has anexceedingly simple construction. Furthermore, with the embodiment ofFIG. 6 the light measuring memory circuit has an exceedingly smalltemperature dependency and alsoan exceedingly small source voltagedependency.

It will be noted that with the embodiment of FIG. 6, as was the casewith FIG. 5 the transistor which is connected to the photovoltaicelement D serves as a photocurrent amplifying transistor. However, withthe embodiment of FIG. 6 it is possible to eliminate the logarithmiccompression element D, and the transistor Q, ofFIG. 's.

In the embodiment of FIG. 7, the photosensitive means is formed by thephotoconductive element CdS, such as a cadmium sulfide light-receivingelement. This photoconductive element of FIG. 7 replaces thephotovoltaic element D and the transistor Q, of FIG. 5. Otherwise theembodiment of FIG. 7 is identical with that of FIG. 5. With thisparticular example during measure ment only of the light enteringthrough the eyepiece, the temperature dependency of the photoconductiveelement is also compensated. Accordingly an exceedingly stable lightmeasurement memory circuit is achieved with an extremely smalltemperature dependency providing an exceedingly great practicaladvantage.

It will thus be seen that with the invention as described above duringthrough-the-lens light measurement, all of the influence of lightentering through the eyepiece is completely compensated as a result ofthe functioning of the electron circuit without special considerationbeing given to the conventional mechanical location or arrangement ofthe light-receiving element or to the method oflight measurement, sothat with the electric shutter control circuit having the memoryfunction, the eyepiece-entering light compensation circuit of thepresent invention can be incorporated in a highly effective manner. Inaddition, as a result of the particular circuit construction shown inthe drawings and described above, the light measurement memory sectionhas an exceedingly small temperature dependency and source voltagedependency, and these ad vantages are achieved in an exceedingly simplemanner. Thus the present invention has great practical advantages.

What is claimed is:

I. In a camera having an objective through which light enters into theinterior of the camera and an eyepiece through which light also iscapable of entering into the interior of the camera, photoresponsivemeans for receiving the light entering into the interior of the camerathrough the objective and through the eyepiece and for converting saidlight into a corresponding first electrical quantity, eyepiece lightmeasuring means for measuring, the light entering into the camera onlythrough the eyepiece thereof and for converting the eyepiece-enteringlight into a second electrical quantity corresponding to theeyepiece-entering light, and electrical circuit means operativelyconnected with said photoresponsive means and eyepiece light measuringmeans for subtracting said second quantity from said first quantity andfor providing an output electrical quantity equal only to the lightentering through the objective, so that said output quantity can be usedin the determination of the extent of exposure of film in the camera.

2. The combination of claim 1 and wherein said photoresponsive means andsaid eyepiece light measuring means respectively include a pair oftransistors wherein the collector current of the transistor of saidphotoresponsive means forms said first quantity and the emitter currentof the transistor of said eyepiece light measuring means forms saidsecond quantity, said electrical circuit means connecting saidtransistors with a pair of conductors for said collector and emittercurrents having a common junction, said circuit means including a thirdconductor for said output electrical quantity also connectedelectrically to said common junction so that said output quantity is inthe form of an output current corresponding only to the light receivedby the objective of the camera, a single photosensitive means beingcommon to and forming part of said photoresponsive means, on the onehand, and said eyepiece light measuring means, on the other hand, saidsingle photosensitive means being a photovoltaic element connectedbetween the base and collector of said transistor of said eyepiece lightmeasuring means and the latter transistor being a photocurrentamplifying transistor common to said eyepiece light measuring means andsaid photoresponsive means, and a switch means also common to saideyepiece light measuring means and said photoresponsive means, saidswitch means being a double-pole double-throw switch having a pair ofblades one of which is permanently connected with the emitter of saidamplifying transistor and the other of which is permanently connected inthe circuit of said photoresponsive means, the latter circuit includinga memory capacitor for maintaining the collector current of thetransistor of said photoresponsive means when said other blade of saidswitch means assumes an open position, said switch means initiallyhaving a position where said blades respectively engage a first pair ofcontacts for connecting said emitter of said amplifying transistor tosaid photoresponsive means and for closing the circuit of saidphotoresponsive means to close the circuit which includes saidcapacitor, and said switch means including only a third contact engagedby the blade of said switch means which is connected to said emitter ofsaid amplifying transistor when said blades are displaced away from saidfirst pair of contacts for opening the circuit which includes saidcapacitor and for closing the circuit of said eyepiece light measuringmeans, said electrical circuit means connecting said third contact tosaid junction for transmitting the emitter current of said amplifyingtransistor to said junction for providing the output currentcorresponding to the difference between the collector current of saidtransistor of said photoresponsive means and the emitter current of theamplifying transistor.

3. The combination of claim 1 and wherein said photoresponsive means andsaid eyepiece light measuring means respectively include a pair oftransistors which are respectively of equal electrical characteristicsbut are of reversed polarities with respect to each other and saidtransistors respectively having collector currents which respectivelyform said first and second quantities, said electrical circuit meansconnecting said transistors in series and including a pair of conductorsfor said collector currents having a common junction, said circuit meansincluding a third conductor for said output electrical quantity alsoconnected electrically to said common junction so that said outputquantity is in the form of an output current corresponding only to thelight received by the objective of the camera.

4. The combination of claim 3 and wherein said photoresponsive means andeyepiece light measuring means respectively include a pair ofphotosensitive means respectively connected electrically with saidtransistors for coacting therewith for respectively providing saidcollector currents,

5. The combination of claim 3 and wherein a single photosensitive meansis common to and forms part of said photoresponsive means, on the onehand, and said eyepiece light measuring means, on the other hand, andwherein a switch means is also common to and forms part of saidphotoresponsive means and said eyepiece light measuring means, saidswitch means having one position for connecting said photosensitivemeans into a circuit of said photoresponsive means for providing thecollector current of said photoresponsive means and another position forplacing said photosensitive means in a circuit of said eyepiece lightmeasuring means for providing the collector current of the latter means,said photoresponsive means and eyepiece light measuring meansrespectively forming a pair of means to one of which said switch meansconnects said photosensitive means first and the other of which saidswitch means connects said photosensitive means second, and memory meansforming part of that one of said pair of means to which saidphotosensitive means is first connected by said switch means forretaining the collector current of said one of said pair of means whilesaid switch means connects said photosensitivemeans to the other of saidpair of means.

6. The combination of claim and wherein said switch means first connectssaid photosensitive means to said photoresponsive means and said memorymeans including a memory capacitor forming part of said photoresponsivemeans for maintaining said collector current thereof while said switchmeans connects said photosensitive means to said eyepiece lightmeasuring means.

7. The combination of claim 6 and wherein said photosensitive means is aphotovoltaic element, said transistor of said eyepiece light measuringmeans also forming part of said photoresponsive means, said transistorof said eyepiece light measuring means being basegrounded and having anemitter electrically connected to said photosensitive means to receiveits input therefrom, said switch means being a double-pole doublethrowswitch having a pair of blades one of which is permanently connectedinto the circuit of said capacitor and the other of which is permanentlyconnected to the collector of said base-grounded transistor, said switchhaving a first pair of contacts respectively engaged first by saidblades and respectively forming part of said circuit of saidphotoresponsive means for providing said collector current thereof, andsaid switch including only a third contact engaged only by the bladeconnected to said collector of said basegrounded transistor when saidblades are displaced from said first contacts to disconnect thecapacitor from said photosensitive means, said third contact beingelectrically connected to said junction through a conductor for saidcollector current of said eyepiece light measuring means so that thelatter collector current is provided through said base-groundedtransistor when said blades are spaced from said first contacts and thecircuit is closed through said third contact.

8. The combination of claim 6 and wherein said switch means is adouble-pole double-throw switch and said photosensitive means is aphotovoltaic element.

9. The combination of claim 8 and wherein said double-pole double-throwswitch has a pair of blades between which said photovoltaic element ispermanently connected, said switch including a first pair of contactsrespectively first engaged by said blades and connected switch means isa single-pole double-throw switch hav-- ing a blade permanentlyconnected electrically to said photosensitive means and having a pair ofcontacts one of which is engaged first by said blade and forms part of acircuit which includes said capacitor and the other of which is engagednext by said blade and forms part of a circuit which includes saidtransistor of said eyepiece light measuring means.

ll. The combination of claim 10 and wherein said eyepiece lightmeasuring means further includes an expansion transistor and alogarithmic compression element.

12. The combination of claim 6 and wherein said photosensitive means isa photovoltaic element, and a photocurrent amplifying transistor commonto and forming part of said photoresponsive means and eyepiece lightmeasuring means, said photovoltaic element being connected between thebase and collector of said amplifying transistor, and a pair oflogarithmic compression elements respectively connected in series withthe emitter and collector of said amplifying transistor, and said switchmeans being a single-pole single-throw switch having a closed positionfor closing the circuit of said photoresponsive means which includessaid capacitor and an open position for providing the collector currentof said transistor of said eyepiece light measuring means.

13. The combination of claim 12 and wherein one of said logarithmiccompression elements is connected across the terminals of said capacitorby said switch when the latter is in its closed position.

14. The combination of claim 6 and wherein said photosensitive means isa photoconductive element.

15. The combination of claim 14 and wherein said switch means is asingle-pole single-throw switch having first a closed position forclosing a circuit which includes said capacitor and then an openposition for opening the latter circuit.

16. The combination of claim 15 and wherein a pair of logarithmiccompression elements are respectively connected in series with saidphotoconductive element with the latter situated between saidlogarithmic compression elements, one ofthe latter elements beingconnected across the terminals of said capacitor when said switch isclosed, and the other of said logarithmic compression elements beingconnected between the base and emitter of the transistor of saideyepiece light measuring means so that when said switch is open thecollector of the latter transistor will provide the currentcorresponding to the eyepiece entering light.

1. In a camera having an objective through which light enters into theinterior of the camera and an eyepiece through which light also iscapable of entering into the interior of the camera, photoresponsivemeans for receiving the light entering into the interior of the camerathrough the objective and through the eyepiece and for converting saidlight into a corresponding first electrical quantity, eyepiece lightmeasuring means for measuring the light entering into the camera onlythrough the eyepiece thereof and for converting the eyepiece-enteringlight into a second electrical quantity corresponding to theeyepieceentering light, and electrical circuit means operativelyconnected with said photoresponsive means and eyepiece light measuringmeans for subtracting said second quantity from said first quantity andfor providing an output electrical quantity equal only to the lightentering through the objective, so that said output quantity can be usedin the determination of the extent of exposure of film in the camera. 2.The combination of claim 1 and wherein said photoresponsive means andsaid eyepiece light measuring means respectively include a pair oftransistors wherein the collector current of the transistor of saidphotoresponsive means forms said first quantity and the emitter currentof the transistor of said eyepiece light measuring means forms saidsecond quantity, said electrical circuit means connecting saidtransistors with a pair of conductors for said collector and emittercurrents having a common junction, said circuit means including a thirdconductor for said output electrical quantity also connectedelectrically to said common junction so that said output quantity is inthe form of an output current corresponding only to the light receivedby the objective of the camera, a single photosensitive means beingcommon to and forming part of said photoresponsive means, on the onehand, and said eyepiece light measuring means, on the other hand, saidsingle photosensitive means being a photovoltaic element connectedbetween the base and collector of said transistor of said eyepiece lightmeasuring means and the latter transistor being a photocurrentamplifying transistor common to said eyepiece light measuring means andsaid photoresponsive means, and a switch means also common to saideyepiece light measuring means and said photoresponsive means, saidswitch means being a double-pole double-throw switch having a pair ofblades one of which is permanently connected with the emitter of saidamplifying transistor and the other of which is permanently connected inthe circuit of said photoresponsive means, the latter circuit includinga memory capacitor for maintaining the colLector current of thetransistor of said photoresponsive means when said other blade of saidswitch means assumes an open position, said switch means initiallyhaving a position where said blades respectively engage a first pair ofcontacts for connecting said emitter of said amplifying transistor tosaid photoresponsive means and for closing the circuit of saidphotoresponsive means to close the circuit which includes saidcapacitor, and said switch means including only a third contact engagedby the blade of said switch means which is connected to said emitter ofsaid amplifying transistor when said blades are displaced away from saidfirst pair of contacts for opening the circuit which includes saidcapacitor and for closing the circuit of said eyepiece light measuringmeans, said electrical circuit means connecting said third contact tosaid junction for transmitting the emitter current of said amplifyingtransistor to said junction for providing the output currentcorresponding to the difference between the collector current of saidtransistor of said photoresponsive means and the emitter current of theamplifying transistor.
 3. The combination of claim 1 and wherein saidphotoresponsive means and said eyepiece light measuring meansrespectively include a pair of transistors which are respectively ofequal electrical characteristics but are of reversed polarities withrespect to each other and said transistors respectively having collectorcurrents which respectively form said first and second quantities, saidelectrical circuit means connecting said transistors in series andincluding a pair of conductors for said collector currents having acommon junction, said circuit means including a third conductor for saidoutput electrical quantity also connected electrically to said commonjunction so that said output quantity is in the form of an outputcurrent corresponding only to the light received by the objective of thecamera.
 4. The combination of claim 3 and wherein said photoresponsivemeans and eyepiece light measuring means respectively include a pair ofphotosensitive means respectively connected electrically with saidtransistors for coacting therewith for respectively providing saidcollector currents.
 5. The combination of claim 3 and wherein a singlephotosensitive means is common to and forms part of said photoresponsivemeans, on the one hand, and said eyepiece light measuring means, on theother hand, and wherein a switch means is also common to and forms partof said photoresponsive means and said eyepiece light measuring means,said switch means having one position for connecting said photosensitivemeans into a circuit of said photoresponsive means for providing thecollector current of said photoresponsive means and another position forplacing said photosensitive means in a circuit of said eyepiece lightmeasuring means for providing the collector current of the latter means,said photoresponsive means and eyepiece light measuring meansrespectively forming a pair of means to one of which said switch meansconnects said photosensitive means first and the other of which saidswitch means connects said photosensitive means second, and memory meansforming part of that one of said pair of means to which saidphotosensitive means is first connected by said switch means forretaining the collector current of said one of said pair of means whilesaid switch means connects said photosensitive means to the other ofsaid pair of means.
 6. The combination of claim 5 and wherein saidswitch means first connects said photosensitive means to saidphotoresponsive means and said memory means including a memory capacitorforming part of said photoresponsive means for maintaining saidcollector current thereof while said switch means connects saidphotosensitive means to said eyepiece light measuring means.
 7. Thecombination of claim 6 and wherein said photosensitive means is aphotovoltaic element, said transistor of said eyepiece light measuringmeans also forming part of said photoresponsive means, said transistorof said eyepiece light measuring means being base-grounded and having anemitter electrically connected to said photosensitive means to receiveits input therefrom, said switch means being a double-pole double-throwswitch having a pair of blades one of which is permanently connectedinto the circuit of said capacitor and the other of which is permanentlyconnected to the collector of said base-grounded transistor, said switchhaving a first pair of contacts respectively engaged first by saidblades and respectively forming part of said circuit of saidphotoresponsive means for providing said collector current thereof, andsaid switch including only a third contact engaged only by the bladeconnected to said collector of said base-grounded transistor when saidblades are displaced from said first contacts to disconnect thecapacitor from said photosensitive means, said third contact beingelectrically connected to said junction through a conductor for saidcollector current of said eyepiece light measuring means so that thelatter collector current is provided through said base-groundedtransistor when said blades are spaced from said first contacts and thecircuit is closed through said third contact.
 8. The combination ofclaim 6 and wherein said switch means is a double-pole double-throwswitch and said photosensitive means is a photovoltaic element.
 9. Thecombination of claim 8 and wherein said double-pole double-throw switchhas a pair of blades between which said photovoltaic element ispermanently connected, said switch including a first pair of contactsrespectively first engaged by said blades and connected to a circuitwhich includes said capacitor and said switch means including a secondpair of contacts engaged by said blades subsequent to engagement of saidfirst pair of contacts, and said second pair of contacts being connectedto a circuit which includes said transistor of said eyepiece lightmeasuring means.
 10. The combination of claim 6 and wherein said switchmeans is a single-pole double-throw switch having a blade permanentlyconnected electrically to said photosensitive means and having a pair ofcontacts one of which is engaged first by said blade and forms part of acircuit which includes said capacitor and the other of which is engagednext by said blade and forms part of a circuit which includes saidtransistor of said eyepiece light measuring means.
 11. The combinationof claim 10 and wherein said eyepiece light measuring means furtherincludes an expansion transistor and a logarithmic compression element.12. The combination of claim 6 and wherein said photosensitive means isa photovoltaic element, and a photocurrent amplifying transistor commonto and forming part of said photoresponsive means and eyepiece lightmeasuring means, said photovoltaic element being connected between thebase and collector of said amplifying transistor, and a pair oflogarithmic compression elements respectively connected in series withthe emitter and collector of said amplifying transistor, and said switchmeans being a single-pole single-throw switch having a closed positionfor closing the circuit of said photoresponsive means which includessaid capacitor and an open position for providing the collector currentof said transistor of said eyepiece light measuring means.
 13. Thecombination of claim 12 and wherein one of said logarithmic compressionelements is connected across the terminals of said capacitor by saidswitch when the latter is in its closed position.
 14. The combination ofclaim 6 and wherein said photosensitive means is a photoconductiveelement.
 15. The combination of claim 14 and wherein said switch meansis a single-pole single-throw switch having first a closed position forclosing a circuit which includes said capacitor and then an openposition for opening the latter circuit.
 16. The combination of claim 15and wherein a pair of logarithmic coMpression elements are respectivelyconnected in series with said photoconductive element with the lattersituated between said logarithmic compression elements, one of thelatter elements being connected across the terminals of said capacitorwhen said switch is closed, and the other of said logarithmiccompression elements being connected between the base and emitter of thetransistor of said eyepiece light measuring means so that when saidswitch is open the collector of the latter transistor will provide thecurrent corresponding to the eyepiece entering light.